Newer epidemiological data have reaffirmed the older concept that the concentration of cholesterol carried in plasma low density lipoproteins (LDL-C) is one of the major causes of the development of atherosclerosis. The steady-state concentration of LDL-C is now known to be determined by four separate variables that include the maximal rate of LDL-C uptake by LDL receptor-dependent transport (Jm), the rate of LDL-C production (Jt), the affinity of the LDL molecule for its receptor (Km), and the rate of LDL uptake through a LDL receptor-independent process (P). Each of these variables is subject to regulation by environmental factors such as dietary cholesterol and fatty acids, and the magnitude of this regulation is also subject to genetic polymorphism. We describe three groups of studies designed to examine these regulatory processes. The first group continues our investigations into the regulation of Jm and Jt by specific dietary fatty acids. Using hamsters fed triacylglycerols containing single fatty acids, a series of measurements will be made in vivo that include rates of lipid and cholesterol absorption, rates of cholesterol synthesis, relative enrichment of hepatic lipids with specific fatty acids, and characterization of changes in Jm and Jt induced by each of these fatty acids. These studies will further test our hypothesis that different fatty acids exert their regulatory effects on LDL-C concentrations by shifting sterol between the hepatic ester pool and the regulatory pool recognized by the nucleus. The second group of studies will examine in more detail the effect of these lipids on LDL-C production. Since one major determinant of the production rate term is the level of hepatic receptor activity, these studies will be carried out in mice with either normal or deleted receptor activity. Detailed measurements made in vivo will then examine the effect of specific fatty acids on the four processes that potentially dictate LDL-C production. These include the level of LDL receptor activity, the concentration of cholesterol in the VLDL particle, the rate of extrahepatic cholesterol synthesis, and the level of cholesteryl ester transfer activity. Having defined how the environmental factors of dietary cholesterol and specific fatty acids regulate the four processes that dictate steady-state LDL-C levels, the third set of studies will use cynomolgus monkeys to examine at what enzymatic or transport step(s) genetic polymorphisms might alter the response of Jm, Jt, Km, or P to challenge by a defined dietary intake of specific fatty acids. Animals will be defined phenotypically as either hyper- or hyporesponders. Groups of these animals will be challenged with a specific set of dietary fatty acids and measurements will then be made of LDL receptor activity and LDL-C production rates. If these prove to be different, then additional measurements will be made of the specific processes that dictate rates of sterol absorption, bile acid secretion, bile acid hydrophobicity and extrahepatic cholesterol synthesis. Taken together, these three groups of studies should provide the first detailed description of the specific effects of individual fatty acids on the cellular processes that dictate steady-state LDL-C levels in the live animal, and should provide important insights into the genetic and cellular mechanisms that articulate the regulation of these levels.